Ion source, circular accelerator using same, and particle beam therapy system

An ion source used for a circular accelerator including magnetic poles facing to each other, and a radiofrequency acceleration electrode, the ion source comprising:

• a microwave emission source provided outside the magnetic poles;
• a waveguide configured to introduce a microwave emitted from the microwave emission source to a region to which a main magnetic field generated by the magnetic poles is applied;
• and
• a magnetic field generation unit provided inside a hole provided in a part of the magnetic poles and configured to generate a magnetic field in a direction opposite to a direction of the magnetic field generated by the magnetic poles, wherein
• plasma is generated inside the magnetic poles by a magnetic field generated by applying the magnetic field generated by the magnetic field generation unit in the opposite direction to the main magnetic field decreased according to a diameter of the hole and the microwave introduced by the waveguide.